PORT-AU-PRINCE, Haiti — Helicopters carrying dozens of American troops landed on the lawn of Haiti’s destroyed National Palace on Tuesday morning, a potent symbol of the United States’ escalating military presence in Haiti since the earthquake that struck a week ago.

With hundreds of Haitians watching and cheering from outside the white-and-green palace gates, troops in combat fatigues bounded out of the helicopters, carrying food rations, bottled water and other gear across the grass, according to photographs and news reports from the scene.

The troops, who appeared to be establishing a position at the palace, were among the roughly 5,000 United States military personnel already in Haiti; thousands more are expected.

American troops took control of the airport in the immediate aftermath of the devastating earthquake, and have been distributing food and water and providing security for the relief effort. But they have, for the most part, not been a major presence on the streets.

Given the long history of American military intervention in Haiti — stretching back to a Marine landing in 1915 — commanders took pains on Tuesday to reassure Haitians that the United States was not invading.

Col. Gregory Kane of the United States Army told reporters at the Port-au-Prince airport — which has come to resemble an American military base, with helicopters coming and going continually — that the Haitian government remained in charge. He said that United States forces were only on the ground to assist in the relief efforts.

“There have been some reports and news stories out there that the U.S. is invading Haiti,” Colonel Kane said. “We’re not invading Haiti. That’s ludicrous. This is humanitarian relief.”

Many Haitians seemed to welcome the promise of help from American troops. In the capital’s Nazon neighborhood, a hand-painted sign on a collapsed building read: “Welcome the US Marines. We need some help.” An arrow on the it pointed into the building, warning, “Dead bodies inside.”

Cmdr. Chris Lounderman of the United States Navy, also at the airport, said that Haitians might not be comfortable with the return of American soldiers and Marines, but he said, “we’re coming in a peaceful posture to deliver aid.”

Other troops are also on the way. The United Nations Security Council on Tuesday unanimously approved sending an additional 3,500 police and peacekeeping troops for Haiti, both to maintain public order and to guard deliveries as the aid effort gathers steam. The forces will augment the 9,000 United Nations troops already here.

So far, violence has been scattered in Port-au-Prince. But senior United Nations officials said it might boil over at any moment as the difficulties of living without water, food and shelter mount.

“We need to be very much careful and vigilant against any possibility,” the United Nations secretary general, Ban Ki-moon, said in an interview in New York. “When their patience level becomes thinner — that is when we have to be concerned.”

Haiti’s capital showed faint signs on Tuesday that life in its most basic form was slowly clawing back amid the chaos and destruction.

The streets of Port-au-Prince contained scenes of commerce and activity Tuesday morning, instead of just devastation and death. Merchants sold fruits and vegetables amid the rubble of destroyed businesses. More cars were winding through the debris-strewn streets.

Women walked with baskets on their heads filled with fruit, cookies an other sundries, functioning as mobile stores. At a homeless encampment at a golf course in the Bourdon neighborhood, impromptu markets have opened up, though prices were high. A woman frying chicken in a pot outside her tent sold each piece for 30 gourde, about $1 — half a day’s wages here. Another woman offered scoops of rice from a large sack balanced on her head, about 30 gourde for a cup and a half, about double the cost before the earthquake.

“It is what I must pay,” the woman, Luciana Delane, explained, saying she had the sack before the earthquake but would have to replace it at increased costs when it runs out. Helicopters buzzed overhead as foreign governments and aid groups tried to coordinate the piecemeal distribution of fresh water, food and medical help.

People continued to stream out of the capital in an uncertain quest for shelter, fresh water and stability in the interior of the country.

There seemed to be no certainty on any front, not even on the death toll. Alain Le Roy, the United Nations peacekeeping chief, said Monday he could not confirm estimates of as many as 200,000 dead.

He said that as far as he knew, the toll had not surpassed 50,000 dead. “I don’t think anybody knows, to be frank,” he told reporters in New York.

For many residents, one clear thing appeared to be the need to leave. Bus after bus lined up at gas stations throughout the city, hoping to fill up with fuel before beginning the long trek out of the earthquake-ravaged zone around the capital. Some people lugged overstuffed suitcases; others carried little more than the clothes they were wearing and enough money to pay the new, higher fares.

At one gas station, the messages on some buses, painted in bright colors above their windshields, evoked something more than hope: Christ Est la Réponse (Christ Is the Answer) and Courage Mon Frère (Courage, My Brother).

“I don’t know if I’m coming back,” said Marcelaine Calixte, 20, a student whose house and college had collapsed, sitting on a crowded bus Monday afternoon headed to Les Cayes, a southern town.

Former President Bill Clinton, the United Nations special envoy to Haiti, arrived in Haiti Monday afternoon and toured the city’s general hospital. “It is astonishing what they’re accomplishing,” Mr. Clinton said afterward, adding that he had been told that the hospital was overwhelmed with patients. They filled its rooms and hallways, and even open areas in the yard outside. Mr. Clinton said he heard of vodka being used to sterilize and of operations performed without lights.

One of the patients outside, Vladamir Tanget, 24, lay on a mattress with a broken leg.

“The government is not doing anything,” he complained. “We need outsiders to come.”

Ginger Thompson and Marc Lacey contributed reporting from Port-au-Prince, James C. McKinley Jr. from Miami, and Neil MacFarquhar from the United Nations.